Probabilities

A probability [that an event occurs] is a number between 0 and 1.

Logical Or

If the probability p_1 for an event occuring is given and the probability p_2 for another event (disjunct from the first) occuring is given, then the probability p of either (or both) occuring is:

p:=p_1+p_2

Logical And

If the probability p_1 for an event occuring is given and the probability p_2 for another event (disjunct from the first) occuring is given, then the probability p of both occuring is:

p:=p_1⋅p_2

If the probability p_1 of an event is known, and this event can only happen after some other event with probability p_2 happened, then the probability p of the event 1 happening is:

p:=÷{p_1⋅p_2}{p_1}

(this is a special case for independent events)